1. Field of the Invention
This invention relates to an improved process for producing metal casting molds using an alkali metal silicate as the binder and a product obtained by reaction of three moles of glycerin and one mole of glacial acetic acid as the catalyst for the binder.
2. Description of the Prior Art
In recent years there has been considerable interest in the production of self setting molds for metal casting from aggregates and liquid alkali metal silicates where the molds are formed in desired shapes and then cured by exposure to carbon dioxide gas. Curing of green molds containing an alkali silicate as the binder involves chemical reaction of carbon dioxide with the alkali metal silicate in the mixture to form a uniformly dispersed silicon dioxide gel binder. There are many disadvantages to this type of curing process. It is difficult to insure satisfactory reaction of carbon dioxide with the alkali metal silicate in the green mold. Curing of a large mold is often difficult. Even when carbon dioxide is injected into several sections of a large mold, some portion of the mold may not receive sufficient carbon dioxide to cure properly. Likewise curing of a small mold is difficult. If the alkali metal silicate in a small mold is reacted with an excess of carbon dioxide, the mold may be too friable. Carbon dioxide curing has the further disadvantage that it requires maintenance of a gas supply and storage in cylinders or tanks.
Other methods of curing silicate binders have also been used. Some of these processes are based on silicate reactions which do not require reaction of the silicate with carbon dioxide. One of these processes is described in U.S. Pat. No. 3,642,503 -- Beaney, issued on Feb. 15, 1972. This process involves the use of either (1) monoacetin or diacetin and (2) triacetin or ethylene glycol diacetate as the catalyst in the silicate reaction. Although this process represents an improvement over silicate processes which use carbon dioxide, there is still a need for further improvements in processes for self setting molds. Beaney reports that reaction of the alkali metal silicate binder with monoacetin or diacetin catalyst is extremely fast and that it is not possible to prepare a mixture of the silicate binder with the monoacetin or diacetin catalyst and shape a mold before the mixture hardens. Further, Beaney reports that use of a mixture of monoacetin and diacetin as the catalyst does not give satisfactory results with a silicate binder.